Ah, life is good! Enjoying a Jacuzzi, a cold beverage, and the warm Caribbean sun, while watching the deep blue water slip by far below, is one of the most relaxing ways to enjoy a vacation. Cruises are hard to beat for convenience too. Unpack only once, visit a different exotic location each day, and indulge in 3-star, 5-course dinners each night. It's not surprising that more and more people are choosing cruise vacations.

As cruising is becoming more popular, ships are becoming larger. The traditional role of the cruise ship as lodging and transportation from port to port is giving way to the concept of the ship itself as the experience. These floating hotels are vying for customers by offering an increasingly broad range of physical activities, restaurants, shopping, and entertainment. In addition, "theme cruises" now target passengers with special interests such as specific music or bands, wine tasting, cooking, history, ecology, or computer science.

The largest cruise ships measure more than 150,000 tons, are over 1000 feet long, boast more than 15 decks, and accommodate more than 4000 paying guests. Each of these gigantic ships requires a crew of more than 1300 to maintain power, electrical, and plumbing systems, cook and clean, provide safety, security, and health services, organize activities, excursions, and logistics, entertain, photograph, print newsletters, work in shops, casinos, and spas, and cater to all the other needs of the passengers. Oh yes, they must also navigate the oceans of the world without getting lost or running into piers, icebergs, or other ships. What does it take to operate one of these behemoths?

Let's start with the basics. How does a ship weighing more than 700 Statues of Liberty float? In the same way as a row boat, it creates buoyancy by displacing a volume of water equal to its mass (or weight). Once a mass of water equal to that of the ship has been displaced, it pushes upward with a force sufficient to support the ship. This force is easily felt by trying to push an air mattress or beach ball under the surface of a pool. The less dense the object the more buoyancy it will create, so ship designers reduce density where possible by using lightweight materials and open spaces.

Buoyancy is also enhanced by designing the ship with a U-shaped hull, which displaces a large volume of water with a relatively shallow "draft," or depth below the surface. The U-shaped hull also contributes to the stability of cruise ships relative to smaller and faster vessels. In addition, two or more underwater stabilizers in the form of large booms (10-15 feet long is typical) may be extended horizontally from the hull on each side to greatly reduce rocking motion, even in high seas.

Safety is paramount on a ship carrying thousands of people. Most are constructed with a double hull, one inside the other, to minimize the likelihood of a catastrophic breach below the waterline. The area below decks is also compartmentalized, so should both hulls be compromised the flooding can be contained in a relatively small volume.

The majority of cruise ships use diesel electric engines to generate power. These consume diesel fuel, as in a diesel automobile, but typically of a lower grade and less clean burning. The engines are used to power electrical generators that in turn drive the motors turning the propellers. For example, the Queen Elizabeth 2 (QE2), a modest 70,000 ton ship, uses nine diesel electric engines to generate 130,000 horsepower, feeding two 400 ton electric motors, one slaved to each propeller shaft.

Some newer ships utilize gas turbine, rather than diesel electric, engines. These burn fuel mixed with compressed air to turn a turbine engine, similar to those in jet airplanes. The engines power electrical generators, which in turn power the motors turning the propeller shafts, just as with the diesel electric systems.

Many ships have engines dedicated only for propulsion with separate engines to satisfy the other electrical power needs of the ship, tantamount to producing electricity for a city of 5000 residents. Most also capture excess heat from engine exhaust to help power boilers producing steam for heating and hot water for showers, laundries, and kitchens.

Both types of engines burn a great deal of fuel, and the high price of oil has become a major cost element for cruise lines. The QE2 moves about 50 feet per gallon of fuel. It consumes an amazing 400 tons of fuel per day, and carries enough for a 10-12 day voyage, depending on distance, speed, and sea conditions. Top speed for many cruise ships is in the neighborhood of 20-25 knots (22-28 mph).

The main drive propellers can be 25 feet or more in diameter, and turn relatively slowly, typically less than 100 rpm. The most modern cruise ships incorporate azimuth thrusters, in which the propellers are mounted in pods that can be rotated 360 degrees horizontally, eliminating the need for rudders and allowing the ship to turn and stop more quickly than traditional designs.

Bow thrusters and stern thrusters are propellers mounted into the side of the ship near the bow and stern. These are reversible so, used in conjunction, they can rotate the ship in place or move it laterally left or right (port or starboard). This is very handy for sliding into or out of a narrow "parking space" along a pier. Ships equipped with azimuth thrusters use these in lieu of stern thrusters.

Satellite navigation systems are available to the ship's crew, including GPS systems that can pinpoint their location anywhere on the planet. However, many navigators rely primarily on the time-tested methods of laying out and tracking a course on paper navigation charts. These are highly detailed maps of the world's seas and navigable waterways.

Cruise ships produce their own fresh water from sea water, using on-board desalination facilities. The largest ships use more than 2000 tons of water (500,000 gallons) a day. This results in a tremendous amount of waste water, including sewage, gray water (from showers, sinks, cooking, etc.), and bilge water, which is likely contaminated with petroleum products. Current laws allow most types of waste water to be discharged in international waters. However, some cruise lines are attempting to find more ecologically-friendly alternatives.

Another ecological issue involves ballast, the sea water a ship takes on in tanks below the waterline to maintain its balance and position in the water. Environmentalists are concerned that as a ship discharges water it collected far away, marine organisms may be introduced to waters in which they previously did not exist, potentially posing a hazard to the ecological balance in the same way that non-native plants and animals can damage land-based ecosystems.

The vulnerability of cruise ship passengers to the spread of viruses has received a lot of media attention. Crowding and close quarters on these ships is a perfect environment to propagate an organism such as the norovirus which causes stomach flu. Crews strive to limit the risk with liberal use of disinfectants and stringent cleanliness rules. And in fact, widespread illnesses on cruise ships are relatively rare.

One of the mostly low-tech wonders of the cruise industry is the manner in which a ship is "turned around." From the time it is secured to the pier at the end of a cruise to the time it sails again is often only 12 hours. In this time, all the passengers and their luggage are removed, the cabins and public areas are cleaned, waste is brought ashore for disposal, the ship is completely restocked with food and supplies, the fuel tanks are filled, any needed maintenance and repairs are performed, and thousands of new passengers are welcomed aboard. This is a marvel of logistics, planning, and choreography.

The challenges of operating cruise ships can only become more difficult as ships continue to get larger. Currently under construction and scheduled to sail in the fall of 2009 is a $1.2 billion, 220,000 ton ship that will carry more than 5500 guests. Hopefully it will have plenty of Jacuzzis.